I recently acquired a Delton log car at a train show.  I want to replace the plastic wheels with Bachmann metal wheels.  Do I need the 24.5mm or 31mm size?  Alsio how is the diameter measured-from the tip of the flange or from the rail contact surface?

I recently acquired a Delton log car that looks great with my Bachmann log cars.  I want to replace the plastic wheels with Bachmann metal wheels.  Which size do I need--the 24.5mm or 31mm?  How is the diameter measured--to the tip of the flange of the rail face surface?